This invention relates to a detector.
One type of fire detector which is in widespread use is the ionization type fire detector. In a detector of this kind an ionization current is exposed to the atmosphere so that if combustion particles are present in the air these will interfere with the ionization current indicating the outbreak or existence of a fire. Certain fire detectors of this type function by comparing the ionization current to a fixed reference value and, when the ionization current crosses the reference value, initiating an alarm.
In this respect the ionization type fire detector functions satisfactorily. However it is subject to certain disadvantages. For example the ionization current is subject to natural drift caused inter alia by variations in temperature, humidity, and dust, and ageing of the radioactive source which produces the ionization current, and can vary to such an extent under the influence of these natural factors that the alarm threshold is crossed even though there is no fire. In addition the ionization current can be affected by malfunctions in the fire detector. It is also responsive to particles, for example dust particles, which are not necessarily combustion particles. Thus for example particles produced during blasting operations in a mine which settle permanently in the ionization type fire detector can cause false triggering.
The specification of UK Pat. No. 1365018 describes a method of distinguishing certain types of false alarms from genuine fire conditions in a two chamber ionization type fire detector. This specification discloses a fire detector which includes a measuring chamber and reference chamber which are connected in series. A voltage is applied across the two chambers and the potential at a point intermediate the chambers is monitored and analysed to distinguish genuine fire conditions and false alarms.
In a fire detector of this type the reference chamber is saturated with ionization current which is therefore essentially constant. The potential at the intermediate point is consequently dependent on the impedance of the measuring chamber which is in turn affected by the presence of combustion products, dust, etc, but it is also dependent on the magnitude of the ionization current which is determined by the characteristics of the reference chamber.
The ionisation current, however, is the physical quantity which is directly influenced by combustion particles, disturbances due to blasting, moisture, or the like, and it is therefore most desirable to monitor the ionization current directly, and to determine that as far as possible the ionization current is affected only by atmospheric conditions and not by equipment parameter variations, to give the most effective analysis of the operation thereof.